A mobile communication network includes a plurality of base stations being connected together by means of switching nodes, such as Base Station Controllers (BSC), Mobile Switching Centres (MSC), and Serving GPRS (General Packet Radio Service) Support Nodes (SGSN). Each base station provides radio coverage over an area known as a cell, for radio communication with mobile terminals located therein. The mobile network has been allowed to use a certain limited radio frequency spectrum for transmissions between base stations and mobile terminals. Thus, data is transmitted over various logical channels on physical frequency channels within the allocated spectrum.
In a typical cellular network configuration, each cell is allocated a number of specified physical frequency channels to be used for call connections and for broadcasting information to mobile terminals. Since the total number of available physical frequency channels is limited, they must be reused to some extent in plural cells throughout the network. However, frequency channels can only be reused in cells being sufficiently distant from each other, as to not interfere too much with each other, although mobile terminals and base stations are adapted to cope with a certain extent of interference.
Cell planners are concerned with allocating frequencies to the cells in networks where certain reuse patterns are employed, which is generally well-known in the art. Great efforts are made to be able to employ tight reuse patterns, i.e., with as short reuse distances as possible, in order to maximize the traffic capacity in the network. An important factor to consider is to keep transmission power levels for calls and broadcasts as low as possible, without jeopardising the radio coverage, in order to reduce the amount of interference, which will in turn enable short reuse distances. Highly accurate power control mechanisms for ongoing calls have therefore been developed.
Another area of interest is the selection of the most appropriate base station for connection with a mobile terminal. For example, if a mobile terminal being connected to a serving base station moves away from that base station towards a neighbouring base station, the received signal strength or link quality of the old base station will decrease and that of the new base station will increase. As a result, the new base station may become more suitable for connection, requiring less transmission power to achieve acceptable link quality.
There are various known mechanisms for switching connection from a serving base station to a new one, referred to as “handover” or “handoff” when the terminal is in busy mode, i.e., engaged in a call, and “cell selection” or “cell reselection” when in idle mode, i.e., not engaged in a call but powered-on. Switching of serving base station may also be performed for mobile terminals being in a packet-switched transfer mode (in contrast to circuit-switched communication). For either mode, this will hereafter be referred to as “base station selection” for simplicity. Hence, correct base station selections will result in low interference and make tight reuse patterns possible, as well as saving battery consumption in the terminals.
In most cellular networks of today, mobile terminals are required to make measurements on signals from neighbouring base stations as well as from the serving base station, and to report the measurement results to the serving base station. Reported measurements can then be used by the network for different purposes, such as:
1) Supporting the process of selecting the most appropriate base station for a specific connection, as discussed above. The reported measurements on signals from neighbouring base stations, as well as from the serving base station, are compared, and the “best” base station is selected for connection, preferably after some predefined threshold condition also has been fulfilled. In the case of handover in busy mode, these measurements are often referred to as MAHO (Mobile Assisted Handover) measurements.
2) Supporting mechanisms for determining the position of the terminal (generally called “positioning”), requiring timing and/or signal strength information of received signals from each target base station. The terminal can measure and report the reception delay and/or signal strength of signals from preferably at least three base stations. From this information and by the known location of each base station, the current position of the terminal can be calculated.
3) Characterising network properties, such as calculating cell relations and evaluating different cell patterns or plans and various algorithms and parameters that are used for operating the network. Various measurements of link quality from different base stations can form a basis for network planning and network configuration work. Cell relations include, e.g., an estimated level of interference if the cells are allocated the same or adjacent frequency channels for transmissions. Typical network planning tasks include: setting cell patterns and transmission power levels, making antenna adjustments and setting frequency allocation parameters and handover thresholds.
From the above, it is evidently desirable that mobile terminals can make accurate and reliable measurements on signals transmitted from different base stations. The impact of reported inaccurate or misleading measurements can thus affect the above-mentioned activities adversely.
In order to enable such measurements, each base station continuously transmits a signal at a fixed power level on at least one broadcast frequency or pilot channel, which in GSM is called the BCCH (Broadcast Control Channel) frequency. Mobile terminals present in the network can then make measurements on the broadcasted signal with respect to, e.g., link quality or timing estimation.
As is well-known, a broadcast frequency includes successive signal bursts transmitted in timeslots. A plurality of logical channels are multiplexed onto the physical broadcast frequency according to a specific TDMA (Time Division Multiple Access) frame structure. The logical channels may include point-to-multipoint channels such as paging channels PCHs, a frequency correction channel (FCCH), a synchronisation channel (SCH), as well as other specific control channels. The logical channels may also include dedicated point-to-point traffic channels (TCH), which can be used for mobile terminal connections, and point-to-point signalling channels, e.g. a Stand alone Dedicated Control Channel (SDCCH).
When a mobile terminal connects to a certain base station, that base station will transmit a measurement order including a list of broadcast channels transmitted by neighbour base stations to be measured by the terminal. Such a neighbour list may be pre-defined for each cell, at least for the purpose of MAHO measurements, and indicates the broadcast frequency of each target base station and possibly also the identity of the base station. The number of base stations included in a neighbour list is typically in the range of 10-32, depending on cell configurations in the network.
To illustrate this measuring procedure, FIG. 1 shows a schematic view of a mobile terminal 10 operating in a cellular communication network 12, including a plurality of base stations 14, 16a-c interconnected by means of a switching network 18. In this case, the terminal 10 is currently connected to a serving base station 14. Initially, when the terminal 10 starts the connection with the serving base station 14, the terminal 10 receives from the network, by means of the base station 14, a measurement order including a neighbour list with the broadcast frequencies (and optionally also identities) of a plurality of predetermined neighbouring base stations, of which three are shown, 16a-c. The terminal 10 is thereby ordered by the network to perform measurements, e.g. with respect to link quality or timing, on the specified broadcast frequencies during idle periods, and to send one or more reports to the serving base station 14 and the network. Such reports may be sent in response to polling from the network, or according to a predetermined schedule either specified by the standard used, or by the measurement order that was sent from the network.
Alternatively, the terminal 10 may as yet be unconnected to any serving base station, like for example when the terminal 10 has just been powered on. The terminal then scans for broadcast frequencies and measures them in order to register with a base station having the strongest/best signal.
As mentioned above, mobile terminals may measure broadcasted signals with respect to link quality or timing. Link quality may be measured as at least one of: a received signal strength (RSS), a carrier-to-interference power ratio (C/I), a carrier power, a Bit Error Rate (BER) or any other link quality related parameter. Timing may be measured by detecting a burst offset in relation to a given clock reference provided from the serving base station. However, the present invention is not limited to any particular measuring methods or schedules.
However, due to the above-mentioned co-channel interference problems, measurement errors may occur if the frequency reuse distances are relatively short. The terminal may for example erroneously measure a strong signal from a base station not being an intended target base station but reusing the same broadcast frequency. Furthermore, the measured signal is typically the sum of plural signals transmitted from several sources reusing the same frequency, including their reflections, and the total measured signal strength and/or quality may therefore be misleading.
In some networks, it is therefore required that the terminal must qualify a measurement by verifying the target base station being measured, before reporting the measurement to the serving base station. By ascribing a certain measurement to a certain base station, that measurement will be more reliable, whereas if the terminal fails to identify the base station, the measurement should be discarded.
In WO 02/096149, a solution is disclosed for mobile terminals to accurately ascribe measurements to specific base stations. The measurements are qualified only when their identity can be determined based on the same received signal being measured.
In order to enable identification of base stations, it has been proposed to include in the broadcasted signal from each base station an indication of that base station's identity which the terminal can read, or at least detect. Base station identification is typically further helped by the terminal knowing, from the received neighbour list, which target base station to expect for each measured frequency. In GSM, a base station identity is used called the Base Station Identity Code (BSIC), which is included in the SCH. The BSIC comprises a Network Colour Code (NCC) and a Base station Colour Code (BCC). Furthermore, normal bursts transmitted on the BCCH frequency may contain information which is related to the BSIC, such that the receiving terminal can derive the BSIC therefrom.
In general, transmitted signals have been more or less corrupted during their propagation, when received by the terminal. A process called channel estimation is therefore typically used by the terminal receiver to recover the transmitted signal. Channel estimation utilizes a training bit sequence known to the terminal, which in the GSM case is 26 bits long, typically embedded in all normal bursts transmitted from base stations, including bursts in the broadcast channel. Exceptions from “normal” bursts may be the FCCH burst which only contains a sinus wave for frequency synchronisation, and the SCH burst which contains a longer specific training sequence used for initial TDMA burst synchronisation. There are numerous known techniques for channel estimation available, which will not be described here further. In timeslots where there is no useful information to transmit, so-called “dummy bursts” containing no data are transmitted, in order to maintain the required continuous transmission on the BCCH frequency.
FIG. 2 illustrates schematically an exemplary normal burst 20 transmitted in a timeslot of a broadcast frequency channel from a target base station included in the neighbour list received by the terminal. The normal burst 20 may belong to any one of many possible logical channels, e.g. a traffic burst belonging to a traffic channel, and includes a bit field 22 with a training sequence, often arranged approximately in the middle of the burst. The burst 20 may further include various fields placed on both sides 24, 26 of the training sequence, such as a header field, fields with payload or control data, tail bit fields etc.
The training sequence in a normal burst from a serving base station is known by the terminal, and is used to facilitate synchronisation and the decoding or detection of the burst. A set of known training sequences have typically been defined, e.g., eight different sequences in GSM, and the one used in a particular burst is identified by a Training Sequence Code (TSC) of, e.g., 3 bits. In a GSM common control channel, the TSC is identical to the BCC, and for other channels, the TSC is communicated in channel assignment messages to the terminal.
According to WO 02/096149, the base station identity is preferably related to the TSC in a way that is known in advance by mobile terminals, by applying “base station specific” training sequences. A mobile terminal receiving a normal burst from a target base station can therefore determine the base station identity, regardless of which logical channel the received burst belongs to, by detecting an expected training sequence in the burst and deriving the TSC therefrom. In the simplest case, the base station identity, such as the BCC in GSM, is set to be identical with the TSC. However, other relationships are possible.
In WO 02/096149, it is further proposed that channel estimation is conducted for measured signals from neighbouring base stations, in order to verify the target base station. Moreover, channel estimation may be conducted for several candidate training sequences, in order to separate contributions from plural base stations reusing the same frequency, as well as any reflections thereof. This procedure will facilitate detection of the target base station, which is described in more detail in WO 02/096149.
In some mobile networks, different modulation methods are used for conveying data bits in the bursts. In networks using EDGE (Enhanced Data rates for GSM/TDMA136 Evolution) technology, i.e. GERAN (GSM/EDGE Radio Access Network), two different modulations are used, namely GMSK (Gaussian Minimum Shift Keying) using two phase positions in each symbol to represent one bit (1 or 0), and 8PSK (8-ary Phase Shift Keying) using eight phase positions in each symbol to represent three bits (1 or 0 each). In GERAN, eight different training sequences with corresponding TSCs have been defined for each modulation form, where each TSC is coupled to one GMSK training sequence and to one 8PSK training sequence, respectively. These training sequences/TSCs have also been stored in mobile terminals capable of operating in GERAN.
Base stations reusing the same broadcast frequency can have different BCCs, and by coupling the TSCs to the BCCs, the training sequences will be different as long as the BCCs are different. This is the case, e.g., in GERAN base stations supporting a positioning technique called Enhanced Observed Time Difference (E-OTD). Hence, if a mobile terminal can recognize the training sequence included in a received burst, the sending base station can be identified from the TSC/BCC derived from that training sequence. Here, the terminal may make an estimation attempt for each possible modulation form on the same received signal, likewise described further in WO 02/096149. However, synchronisation bursts, frequency correction bursts and dummy bursts on the broadcast frequency have no base station specific training sequences, but all other logical channel bursts should contain one of the above-described base station specific training sequences.
At least for measurements intended for base station selection purposes, it is important that each base station continuously transmit on the broadcast frequency, even in timeslots that are currently unused (i.e. where there is no data to transmit), to enable measurements at any time. Unused timeslots may occur in the broadcast frequency frame structure, e.g., when there are unoccupied traffic channels due to low traffic load, or when so-called discontinuous transmission (DTX) is currently applied for an occupied traffic channel, which is a well-known algorithm for minimising interference. In DTX, no data is transmitted when not necessary, e.g. due to silence at the sender side. In cellular networks, it is a general practice to transmit dummy bursts on broadcast frequency channels in unused timeslots, as mentioned above, in order to maintain continuous transmission.
Normally, a “traditional” dummy burst carries no intelligible information, and a predetermined and fixed bit pattern is typically transmitted in all dummy bursts, easily recognizable to mobile terminals. Therefore, it is not possible to determine any base station identity from such dummy bursts, which may sometimes occur quite frequently. Synchronisation bursts and frequency correction bursts occur at fixed frame positions known by mobile terminals, but the occurrence of dummy bursts is more or less unpredictable to them. Moreover, the fixed bit pattern in the entire burst requires special treatment by mobile terminals, since the normal training sequence is not included as expected. Hence, the occurrence of dummy bursts have rendered measurements on neighbouring base stations more difficult.
To overcome these problems, it has been proposed to include in dummy bursts transmitted over broadcast frequency channels, an indication of the base station identity, such as a training sequence which is related to the base station identity in a known way. This will therefore significantly increase the possibilities for mobile terminals to determine the identity of the neighbouring base station having sent a received dummy burst, and to make successful measurements to support, e.g., base station selection and positioning activities.
If a known base station specific training sequence is included in a received dummy burst, the terminal can use it to identify the base station as well as to, e.g. in E-OTD, estimate the timing offset to the corresponding base station, or better estimate the link quality of the burst.
Even though dummy bursts with base station specific training sequences would enable identification of neighbouring base stations, another problem will arise in that the dummy bursts could be erroneously interpreted by mobile terminals as their regular traffic bursts in some situations. For example, if a mobile terminal is active in a call, but the used downlink traffic channel is currently in DTX mode, i.e. the corresponding time slots are unused, the base station must transmit dummy bursts instead to satisfy the continuous broadcasting requirement. Mobile terminals can easily distinguish normal traffic bursts from the traditional dummy bursts with their specific bit pattern, which has very low cross correlation with the training sequences in regular traffic bursts, and correctly interpret such dummy bursts as a continued DTX mode. On the other hand, if a transmitted dummy burst would include a base station specific training sequence instead, it could potentially be erroneously interpreted by the terminal as a regular traffic burst having the same training sequence.
If one or more such dummy bursts are interpreted as traffic bursts, the terminal may erroneously believe that the DTX mode is finished and that a speech frame has been received. In that case, the terminal will attempt to decode the error correcting code of the supposed speech frame. Further, it will attempt to decode an error detecting code (also known as a Cyclic Redundancy Check (CRC) code) of the speech frame, to check whether it is a valid speech frame. Although the error detecting code in most cases gives a correct indication of whether a speech frame is valid or not, it is still possible that an invalid frame is incorrectly deemed to be valid. The probability that an invalid speech frame is interpreted as valid depends on the number of bits used for the CRC. Typically, this probability decreases exponentially with an increasing number of CRC bits. If an invalid speech frame is delivered to the speech decoder in the terminal, this results in unwanted noise. This problem is especially relevant for the TCH/FS (Traffic Channel/Full rate Speech) and TCH/HS (Traffic Channel/Half rate Speech) channels, carrying speech frames adapted to the early speech codecs of GSM, where only three CRC bits are used. A three bit CRC means that approximately ⅛-th of the invalid frames are interpreted as valid by the CRC.
The potential problem of misinterpreting traffic bursts would most likely supersede the benefits gained for the identification and measuring activities, and therefore has to be taken into account when introducing dummy bursts with base station specific training sequences.
To conclude, it is of great importance to obtain reliable measurements on neighbour base stations with high accuracy, and to minimise the delay time between such measurements and corresponding reports, in order to improve procedures of base station selection, position determination and network evaluation. At the same time, it is desirable to avoid, as far as possible, the risk of misinterpretation of different bursts received by mobile terminals. It is also desirable to reduce the impact of interference on the measurement accuracy, thereby allowing for a tighter frequency reuse.